Re-open PR205 MegaETH charge alignment

[ifavo]

Accidental merge note: PR #7 was merged to main by mistake and then reverted in 29c51f5. This draft PR keeps the same work pending for later review and merge.

Summary

• align megaeth/charge request parsing and client/server credential handling with PR 205, including testnet precedence, ordered single-leg Permit2 authorizations[], and split-hash rejection
• settle split Permit2 payments sequentially on the server, add fee-payer simulation plus split-failure diagnostics, and preserve the current recipient/spender coupling as a documented PR 205 gap
• override the default MegaETH HTTP receipt transport to include challengeId, and update tests/docs/demo notes to explain the remaining upstream mppx receipt-schema limitation

Validation

• pnpm ci:check

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	mega-mpp-demo-worker	22aff94	Mar 28 2026, 11:11 AM